Electrochemical apparatus with reactant micro-channels

ABSTRACT

The present invention is directed generally to an electrochemical apparatus for oxidation or consumption of a fuel, and the generation of electricity, such as, a solid electrolyte fuel cell. The electrochemical apparatus ( 1 ) comprises at least one cell ( 2 ), wherein the cell ( 2 ) has a solid electrolyte ( 10 ) disposed between a unitary oxygen electrode ( 12 ) and a unitary fuel electrode ( 8 ), and at least one separator ( 6 ) contacting the surface of one of the electrodes ( 13 ) opposite of the electrolyte ( 10 ). At least one electrode ( 13 ) of the cell ( 2 ) defines a micro-channel pattern ( 26 ), wherein the micro-channel cross-section is preferably varied, such that reactant gas flowing through the micro channels achieves tailored local flow, pressure, and velocity distributions.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation-in-part of copendingapplication U.S. Ser. No. 09/455,149, filed on Dec. 6, 1999 (allowed)which is incorporated herein by reference.

TECHNICAL FIELD

[0002] The present invention relates to fuel cells, and moreparticularly to fuel cells constructed of stacked plate components. Moreparticularly, the present invention relates to fuel cells containingenhanced flow electrodes for fuel and/or air.

BACKGROUND OF THE INVENTION

[0003] The invention is directed generally to an electrochemicalapparatus for oxidation or consumption of a fuel, and the generation ofelectricity, such as, a solid electrolyte fuel cell.

[0004] Although particular embodiments are applicable to conventionalco-fired solid electrolyte fuel cell apparatus, the present invention isparticularly useful when utilizing non-cofired solid oxide electrolytefuel cells, preferably planar fuel cells, that contain a stack ofmultiple assemblies. Each assembly comprises a solid electrolytedisposed between a cathode and an anode, being bounded by separators,which contact the surfaces of the electrodes opposite the electrolyte.

[0005] The fuel cell operates by conducting ions through theelectrolyte. For solid oxide fuel cells in particular, oxygen or air isintroduced at the cathode, and ionization of oxygen occurs at thecathode/electrolyte surface. The oxygen ions move across the gasnon-permeable electrolyte to the anode interface, where it reacts withthe fuel flowing into the anode at the anode/electrolyte interface,releasing heat and supplying electrons to the anode. Distribution of theair and fuel reactants is typically performed by a manifold assemblywithin the fuel cell apparatus.

[0006] Conventionally, each reactant is supplied through a flow conduitto the appropriate electrode, and distribution to theelectrode/electrolyte interface is accomplished by internal porosityand/or grooved channels.

[0007] Minh, U.S. Pat. No. 5,256,499, discloses a monolithic fuel cellhaving an integrally formed manifold constructed by corrugations formedwithin the anode and cathode with aligned ribs and columns arranged toforce fuel and oxidant along aligned pathways. Reactants are fed fromthe sides of the fuel cell and travel along these pathways.

[0008] Hsu, U.S. Pat. No. 5,747,485, discloses a conductor plate for asolid oxide fuel cell with ridges extending therefrom. These ridges formgrooves used to channel reacting gases out of the cell.

[0009] Datta, U.K. Patent No. 2,219,125A discloses an electrolyte with athree-dimensional groove arrangement used to control hot spots withinthe electrolyte block.

[0010] Hsu, Minh and Datta employ external manifolding and rectangulargeometries driving the reactants from one side of the cell to the other.Despite the use of channels, reactants entering from a single side ofthe cell deplete as they travel across the cell. Further, when reactantsare fed externally from more than one side, the flows converge creatinglocalized areas of increased reaction. The increased number of reactionsgenerates an undesirable thermal gradient, which can damage the cell.

[0011] Moreover, Hsu, Minh and Datta employ grooves of uniform crosssection along the length of these grooves. These grooves are essentiallypathways within the cell, and fail to control gas flow rate or pressuredistribution. The flow rate is controlled at its source and not tailoredor controlled within the cell.

[0012] In fuel cells which have their anode fuel-exit edges exposed toan oxidizing environment, any anode local exit regions having low fuelmixture velocities may allow oxygen back diffusion into the cell stack,causing premature combustion and loss of active anode area. Theelectrochemical processes inherent in the fuel cell's operation becomeless effective and performance suffers.

[0013] Custom flow pattern design is desirable to achieve substantiallyuniform reactant concentration distribution within the cell and fromcell to cell within a stack, which also helps minimize unnecessary andundesirable thermal gradients within the cell.

[0014] It is desirable to provide a compact, centrally fed radial fuelcell utilizing micro-channels to tailor the flow distribution ofreacting gases within the fuel cell and amongst all the cells in astack.

[0015] It is also desirable to provide a compact fuel cell utilizingvariable cross-section micro-channels to tailor the flow, pressure, andvelocity distribution of reacting gases within the fuel cell and amongstall the cells in a stack.

[0016] It is also desirable to provide an enhanced flow electrodeproduced by simple scalable production techniques.

SUMMARY OF THE INVENTION

[0017] We have found that micro-channels integrated within the electrodestructure can be formed in a compact fuel cell. Integratedmicro-channels minimize the complexity of stack components. Channels ofsmaller dimension than those existing in the prior art can bemanufactured by a variety of techniques. Using these techniques, flowand pressure distribution can be customized and controlled through thechannel design, enhancing reactant distribution to the cell. It hasfurther been found that a fuel cell apparatus employing a network ofmicro-channels can improve overall cell reactant balance throughcontrolled pressure distribution. It has further been found thatemploying controlled flow and pressure in a compact integrated deviceresults in an apparatus exhibiting improved volumetric power density andefficiency.

[0018] The present invention therefore provides an electrochemicalapparatus comprising at least one cell, wherein the cell has a solidelectrolyte disposed between an oxygen electrode and a fuel electrode,with at least one separator between adjacent cells contacting thesurface of one of the electrodes opposite the electrolyte; wherein atleast one electrode of the cell defines a variable cross-sectionmicro-channel pattern, wherein this pattern serves to distribute theflowing gas uniformly within the electrode, regulates the pressure dropof this gas, and also creates preferred local gas velocities, especiallywhere the gas exits the electrode.

[0019] The present invention further provides an electrochemicalapparatus comprising at least one cell, having a solid electrolytedisposed between an oxygen electrode and a fuel electrode; and at leastone separator contacting the surface of one of the electrodes oppositethe electrolyte. In one embodiment, at least one separator preferablydefines a micro-channel pattern; wherein the micro-channel patternnarrows towards the cell rim, such that gas flowing out the rim isaccelerated.

[0020] The micro-channel is preferably a small size, on the order ofabout 0.5 millimeter or less, such that the micro-channel can be definedwithin at least one electrode or separator by low-cost manufacturingtechniques.

[0021] The present invention also provides an electrochemical apparatuscomprising an electrode defining a pattern of micro-channels fordirecting the flow of reactant; wherein the cross sectional area of themicro-channels is varied along the micro-channel length.

[0022] The present invention also provides an electrochemical apparatuscomprising a plurality of cells forming a stack; each cell within thestack has a solid electrolyte disposed between an oxygen electrode and afuel electrode, with at least one separator contacting the surface ofone of the electrodes opposite the electrolyte. In substantially each ofthese cells, at least one electrode defines a variable cross-sectionmicro-channel pattern.

[0023] The present invention also provides an electrochemical apparatuscomprising at least one cell having a solid electrolyte disposed betweenan oxygen electrode and a fuel electrode, and at least one separatorcontacting the surface of one of the electrodes opposite theelectrolyte; wherein at least one electrode or the electrolyte or theseparator surface has a plurality of columns extending therefrom; saidcolumns defining variable cross-section micro-channels therebetween.

[0024] The present invention also provides an electrochemical apparatuscomprising at least one circular cell having a cell rim; said cell has asolid electrolyte layer disposed between an oxygen electrode layer and afuel electrode layer; at least one separator layer contacting thesurface of one of the electrodes opposite the electrolyte; wherein eachof the layers define at least one air hole and at least one fuel holeand wherein the respective holes in each layer are registrable with oneanother and define generally central internal air and fuel manifolds;wherein at least one layer has a plurality of circular columns extendinglongitudinally outwardly from the respective air or fuel manifold,defining a micro-channel pattern. Preferably, the columns are arrangedin radially expanding rows; and an increasing number of columns extendfrom said at least one layer in each of said rows, such that saidcolumns define a variable cross-section micro-channel that narrowstoward the cell rim.

[0025] The present invention also provides an electrochemical apparatuscomprising at least one fuel cell, wherein the cell has a solidelectrolyte disposed between an oxygen electrode and a fuel electrode,and at least one separator contacting the surface of one of theelectrodes opposite the electrolyte; wherein the cell defines at leastone air manifold and at least one fuel manifold located substantiallycentrally within the cell; and at least one of the electrodes defines amicro-channel pattern.

[0026] The present invention also provides an electrochemical apparatuscomprising at least one cell; wherein the cell has a solid electrolytedisposed between an oxygen electrode and a fuel electrode, wherein atleast one said electrode is a unitary electrode, at least one separatorcontacting the surface of one of the electrodes opposite theelectrolyte; wherein the at least one electrode defines a variablecross-section micro-channel pattern adapted to control flow distributionand pressure drop of at least one reactant gas.

[0027] The present invention further provides an electrochemicalapparatus comprising at least one cell, having a solid electrolytedisposed between oxygen electrode and a fuel electrode; wherein at leastone said electrode is a unitary electrode; and at least one separatorcontacting the surface of one of the electrodes opposite theelectrolyte; wherein the at least one separator defines a pattern ofmicro-channels; wherein the micro-channel cross-sectional area narrowstoward the cell rim adapted to control flow distribution and pressuredrop of at least one reactant gas.

[0028] The present invention also provides an electrochemical apparatuscomprising a unitary electrode defining a pattern of micro-channels,wherein the electrode has a rim; and said micro-channels have across-sectional area that decreases towards the electrode rim for thecontrolled flow distribution and pressure drop of at least one reactantgas.

[0029] The present invention further provides, in a process for thefabrication of a solid oxide fuel cell comprising at least one cellhaving a cell rim, wherein said cell has a solid electrolyte layerdisposed between an oxygen electrode layer and a fuel electrode layer,and at least one separator layer contacting the surface of one of theelectrodes opposite said electrolyte; wherein each of the layers defineat least one air hole and at least one fuel hole and wherein therespective holes within each layer are registerable with one another anddefine generally central internal air and fuel manifolds; theimprovement including providing reactant micro-channels in at least onelayer, said micro-channels having a width of not more than about 0.5 mm.

[0030] The micro-channel patterns may be fabricated by a variety ofknown fabrication methods. One preferred method is the use of mechanicalpressing.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031]FIG. 1 is a partially schematic, partially exploded side view offuel cells capable of having an enhanced flow micro-channel pattern inone of the layers according to the present invention.

[0032]FIGS. 2A and 2B are plan views of enhanced flow micro-channelcontaining electrodes according to the present invention.

[0033]FIG. 3 is a partially schematic sectional side view of a cellaccording to the present invention as seen along line 3-3 in FIG. 2.

[0034]FIG. 4 is a partially schematic sectional side view of an enhancedflow electrode containing micro-channels according to another embodimentof the present invention.

[0035]FIG. 5 is a partially schematic sectional side view of a crossflow layer channel according to the prior art.

DETAILED DESCRIPTION OF THE INVENTION

[0036] Although applicable to other types of electrochemical apparatus,for purposes of this description the invention will be described inrelation to its incorporation into a solid electrolyte (oxide) fuel cellas described in U.S. Pat. No. 5,445,903, incorporated by reference as ifreprinted herein. The electrochemical apparatus 1 of one embodiment ofthe present invention is represented in FIG. 1, which shows a schematicexploded view of one preferred embodiment of a solid-oxide fuel cell 2and a stack of two such cells 4.

[0037] A cell 2 generally comprises four stacked layers: a separator 6,a cathode layer 8, an electrolyte 10, and an anode layer 12. Cathodelayer 8 and anode layer 12 may be referred to in the general sense aselectrodes 13 and in a preferred embodiment at least one such electrodeis comprised of a unitary (single) component as contrasted to an activeelectrode structure and an associated porous element. A tubular gasket14 in a cathode layer forms a seal between the nonporous separator andelectrolyte. A pair of tubular gaskets 16 in the anode layer form sealsbetween the electrolyte and separator. Gaskets 14 and 16 must remainimpervious to fuel and air respectively at the relatively high operatingtemperature of the cell and must be capable of maintaining a good sealunder cell operating conditions. Suitable gaskets 14 and 16 can be madefrom oxidation resistant metal alloys such as nickel-base alloys, fromceramics, or from glasses or glass-ceramics having suitable softeningtemperatures.

[0038] As shown in FIGS. 1 and 3, the separator contains an internalfuel hole 18, which is aligned with corresponding holes in the othercell layers to form an internal fuel manifold 19. It also contains apair of internal air holes 20, which are aligned with correspondingholes in the other cell layers to form a pair of internal air manifolds21. It is within the scope of the invention to include single ormultiple fuel passages and/or oxygen passages in various cell locations,preferably close to the centerline of the cell.

[0039] A suitable hot fuel gas mixture 22, represented by an arrow, isfed to the internal fuel manifold 19 and hot air 24, represented byarrows, is fed to both internal air manifolds 21. The stack of fuelcells will typically operate at about 850° to 1000° C., but may operateas low as 600° C. with suitable low-temperature solid electrolytes.

[0040] The separators 6 must be impervious to gases, be good conductorsof electrons, and have long-term compatibility with both the adjacentmaterial and with the air and fuel mixtures. They should also be fairlygood conductors of heat. Suitable materials include doped lanthanumchromite or high-temperature metallic alloys, such as RA330, Ducralloy,Inconel 601, or Haynes 230 available from Rolled Alloys, Plansee, IncoAlloys International, and Haynes respectively.

[0041] The porous cathode layer or oxygen electrode 8, is generally madeof a mixed oxide preferably such as strontium-doped lanthanum manganite(LSM). The electrolyte 10 is impervious to gases and is a good oxygenion conductor while having little or no electronic conductivity.Yttria-doped zirconia having about 6 to 10 mole percent Y₂O₃ ispreferred. The electrolyte 10 is preferably coated with a thin, firedlayer of LSM on the cathode side and nickel oxide/doped ceria on theanode side.

[0042] The porous anode layer or fuel electrode 12 is preferably made ofnickel felt, nickel-zirconia cermet, or other nickel-containing cermetor alloy.

[0043] Cell and stack diameters are typically about 50 to about 80 mmand total cell thickness (in use) is typically about 1 to about 2 mm,but can be of slightly larger diameter.

[0044] When the cells 2 are stacked, a series electrical connection isestablished among all the cells in the stack, such that the stackvoltage is the sum of all the cell voltages. In use, in one embodiment astack is clamped between a pair of high-temperature electrical contactblocks equipped with mating holes for feeding gaseous fuel and air viafeed tubes (not shown). At one end of the stack, the separator 6 isomitted and, thus, the stack is bounded by a cathode layer 8 at one endand an anode layer 12 at the other end. The fuel gas and air may be fedinto opposite ends or the same ends of the stack.

[0045] The stack is operated by preheating the apparatus close tooperating temperature, supplying air and fuel gas, and connecting anexternal electric load. Oxygen from the air is ionized at, or near, thecathode-electrolyte interface. The oxygen ions flow through theelectrolyte under the influence of the chemical potential difference.At, or near, the electrolyte-anode interface the oxygen ions combinewith fuel molecules (chiefly hydrogen and carbon monoxide), releasingelectrons which flow into the next cell. Typical power densities are onthe order of about 150 mW/cm² of electrode area at typical celloperating voltages near about 0.7 volts. Typical stack volumetric powerdensities are close to about 1.0 kilowatt/liter.

[0046] The cathode layer 8 is preferably a porous body having athickness in the range of about 0.2 to about 0.6 mm, and composed ofconventional cathode material, most preferably an oxide having theperovskite crystalline form such as strontium doped lanthanum manganite(LaMnO₃), doped calcium manganite (CaMnO₃), lanthanum chromite (LaCrO₃),lanthanum cobaltite, (LaCoO₃), lanthanum nickelite (LaNiO₃), lanthanumferrite (LaFeO₃), or mixtures thereof. The cathode 8 may comprise mixedionic/electronic conductors such as an appropriately doped perovskiteoxide listed above. The cathode 8 can be prepared by conventionalceramic processing procedures for making a flat, planar structure,including pressing a powder, or extruding or tape casting a green body,and sintering either prior to or during the initial operation of theapparatus.

[0047] Electrolyte 10 is a thin wafer, generally less than about 0.4 mmthick, preferably about 0.2 mm or less of conventional solid oxide fuelcell electrolyte material. Representative electrolytes include zirconia(ZrO₂) stabilized with 6 to 10 mole percent of yttria (Y₂O₃), dopedcerium oxide, doped bismuth oxide, and oxide ion conducting perovskites.Electrolyte 10 is substantially impervious to gases, however, ionizedoxygen can migrate through the electrolyte under the influence ofapplied oxygen potential.

[0048] The quality of the electrical contact between the cathode 8 andthe electrolyte 10 may be improved by initially applying a thin layer ofsubstantially the material that comprises the cathode 8 (or is at leastelectrochemically compatible with the cathode) to the surface of theelectrolyte 10 adjacent the cathode 8 in the form of a paint or inkincluding a volatile vehicle to form an electrical contact zone.Likewise, a paint or ink containing substantially anode material such asnickel or nickel oxide may be applied to the surface of the electrolyteadjacent the anode to form such an electrical contact zone. Thiselectrolyte surface coating may be applied by other conventionaltechniques also, such as plasma deposition, spin casting, spraying, orscreen printing.

[0049] The thickness of the electrolyte surface coatings is generally onthe order of about 1 to less than about 100 microns, and preferably lessthan 50 microns. It has been found that the thicker this surface coatingis applied, the less gas is able to contact the electrolyte 10, and themore tendency there is for the coating to peel off. Unless specificallystated to the contrary, the electrolyte 10 as mentioned in thisSpecification shall mean the electrolyte 10 with or without either orboth cathode and anode material surface coatings.

[0050] Anode 12 is a porous body, and may comprise conventional solidoxide fuel cell anode material. Preferably, the anode comprises eithernickel felt or else a finely divided, compressed metallic powder such asnickel blended with a stable oxide powder such as zirconia, cation-dopedceria. As described above regarding the cathode 8, the anode 12 maycomprise a mixed conductor, optionally combined with an electronicallyconducting material. Other examples include ceria, which can be dopedwith an oxide of lanthanum, zirconium or thorium, optionally containingan electronically conducting phase such as Co, Ru, or Pt. The thicknessof the anode is preferably about 0.1 mm to about 0.5 mm. Like cathode 8,anode 12 may be sintered during cell operation or before initialoperation in an overheating sintering step.

[0051] In the preferred embodiment as shown in FIG. 2A, at least oneelectrode 13 defines a plurality of micro-channels 26, as necessary. Inthe alternative, the separator 6 might define the micro channels 26 oneither or both of its surfaces. The narrowing of the micro-channels 26is not linear based on the distance from the flow source in the cellelement but rather narrow non-linearly progressively towards the cellrim 32, such as a function r^(n) where r is the distance along thechannel toward the cell rim and n is a number greater than 1, thatdepends upon the system parameters. Since the separators contact theanode and cathode surfaces, micro-channels 26 defined within theseparator surfaces would also provide reactant channeling. For sake ofsimplicity, the description, while referring to electrodemicro-channels, encompasses micro-channels formed within the separator 6as well.

[0052] As shown in FIG. 2A, micro-channels 26 may be formed within anelectrode 13. These micro-channels 26 create a preferential path forreactant flow across the electrode 13. As shown, in simplified form, amicro-channel 26 may be defined by a quantity of regularly spacedcircular columns 34 extending between surfaces of adjacent layers.(Although circular columns are preferred, columns of other geometriesmay be utilized to provide customized flow characteristics.) The spacesbetween the columns 34 provide a preferential path for gas flow. Usingcathode 8 as an example, air enters the micro-channel 26 from internalair manifold 21 via air holes 20. Gaskets or seals 14 isolate the airfrom fuel manifolds 19 and fuel hole 18 formed within cathode 8. Theentering gas spreads outwardly amongst the columns 34 of electrodematerial, successively passing the columnar rows from inner row 36 toouter row 38 before exiting at the rim 32. It should be understood thata preferred pattern of columns 34 would utilize many more columns thanshown in the simplified figure, with each column having a diameter onthe order of about 1 mm or less. The height of each column 34 isgenerally on the order of about 0.05 mm to about 0.4 mm, preferablyabout 0.1 mm. It should be appreciated that the depth of micro-channels26 may comprise substantially the entire thickness of the electrode 13.

[0053] The preferred pattern may be designed to control flowdistribution within a cell 2 by defining pathways that offer reducedresistance in comparison with the surrounding material. The flowdistribution may be further controlled by the number, size, orarrangement of the micro-channels 26 within the cell 2.

[0054] The preferred pattern is designed with consideration to thecolumn spacing and the contact-area percentage. Column spacing may berelatively wide to help minimize the cell pressure drop. Pressure iscontrolled by the size of the column (diameter) and the number ofcolumns per square centimeter. The column diameter and the contact-areapercentage may be selected by a compromise between minimizing electricalresistance, achieving good reacting gas distribution to and from theactive electrode sites, achieving the target pressure drop within aminimum pattern thickness, and fabrication limitations, if any.

[0055] The pattern may be designed to achieve a specific overallpressure drop at its design gas flow rate. It is also possible tomanufacture a pattern with a desired lack of symmetry, to account forany expected side-to-side temperature difference within the stack, forexample. Both the column shape and pattern layout may vary to producethe desired result. While the columns are shown in the Figures to be ofa circular cross section, it is within the scope of the invention thatthe columns be formed with other cross sectional shapes, such as ovals,squares, rectangles, and other regular or irregular polygonal shapes. Itshould further be understood that in addition to columnar patterns,continuous channels may be formed within electrode 13 including gridchannels, spiral channels, and radial line channels. The distribution offlow and achievement of a desired pressure drop may be controlled byusing these types of channels as described above.

[0056] At the stack level, the flow distribution along the length of thestack may similarly be controlled by varying the number, size, anddistribution of micro-channels 26 in different cells in accordance withthe desired stack-wide distribution of reactants.

[0057]FIG. 2B is a simplified schematic illustration of an example fuelelectrode 12 micro-channel pattern with variable cross-section flowchannels formed on a separator 6. The pattern consists of a quantity ofcircular posts or columns 34 with open spaces between them where the gasflows. The fuel gas is fed into the micro-channel 26 pattern from fuelmanifold 19 via a fuel hole 18. Seals 16 isolate the fuel from airmanifolds 21 and air holes 20. The gas flows outwards amongst thecolumns, first passing the inner row of columns 36 and finally the outerrow of columns 38 before exiting at the rim 32. The preferred patternwould utilize many more columns than shown in this simplified figure,with each column having a diameter on the order of about 1 mm or less.The preferred height of each column is very short, on the order of about0.1 mm.

[0058] Using variable cross-section micro-channels, the preferredpattern would be designed using several considerations as follows. Thecolumn spacing would be relatively wide near the center of the cell,where the gas flow diameter is small, to help minimize the cell pressuredrop. The spacing would be relatively narrow near the rim of the cell inorder to achieve a good gas exit velocity, thereby preventing thesurrounding gas mixture from diffusing backwards into the cell. Thediameter of the columns and their contact-area percentage based on thearea of the adjacent layer would be selected as a compromise betweenminimizing electrical resistance, achieving good reactant gasdistribution to and from the active electrode sites, achieving thetarget pressure drop with a minimum pattern thickness, and fabricationlimitations, if any. If the inner row of columns were arranged in acircular pattern as shown, good circumferential symmetry of gas flowcould be achieved even when the center cavity is non-circular.

[0059] The pattern may be designed to achieve a specific target overallpressure drop at its designed gas flow rate. It would also be possibleto manufacture a pattern with a desired lack of circular symmetry, if sowished due to an expected side-to-side temperature difference of thestack, for example. Both the column shape and the pattern layout couldvary in many different ways as still be able to produce the desiredresults. Additionally, the thickness or height of the pattern might bevaried from center to rim as another means for tailoring local flow,pressure, and velocity. It should be understood that micro-channels 26may comprise substantially the entire thickness of the electrode.

[0060] It should further be understood that in addition to columnarpatterns, continuous channels may be formed within electrodes 13. Someexamples include, grid channels, spiral channels, and radial linechannels. In a manner similar to the patterns, the flow, pressure, andvelocity of reactants may be controlled by varying the cross-section ofthese channels.

[0061] The micro-channels 26 may be fabricated into the surface ofelectrode 13, electrolyte 10 or separator 6 by a variety of conventionalsubtractive techniques including electrical-discharge machining,stamping, laser ablation, chemical etching, ultrasonic etching,scribing, and grinding. As a benefit of the present invention, themicro-channels 26 may be formed by photolithography, pressing,calendering, micro electro mechanical systems (MEMS) techniques, oradditive deposition techniques, air brush painting, stenciling, orscreen printing. MEMS techniques include microetching, and micro- ornano-machining. The use of these techniques is possible because of theelectrode 13 and micro-channel 26 size. The micro-channels may be formedby additive or subtractive techniques as set forth above, as applied toan electrode, electrolyte or separator. Material can be removed from thesurface of one of the layers to provide the micro-channel, or materialcan be added to the surface of at least one of the layers. For example,electrode material can be deposited on the electrode, or the adjacentseparator or electrolyte surface, to form columns which define themicro-channels as the space therebetween.

[0062] In the electrodes, the pillars 34 or micro-channels 26 arepreferably made by uniaxially pressing a pattern into an unfiredelectrode preform. This preform is made of electrode powder or premixedceramic-metallic powders mixed with an organic binder material. Thiscombination of components is processed into a soft, ductile mixturehaving a dough-like consistency that can be easily pressed into avariety of shapes. The mixture is sufficiently rigid, however, to retainany impressed pattern including columns 34 and micro-channels 26.

[0063]FIG. 4 depicts a porous electrode 13 having micro-channels 26formed between columns 34 of electrode material. The width of themicro-channels is generally on the order of about 0.1 to about 0.5 mm,and the depth of the micro-channels is generally on the order of about0.1 to about 0.5 mm, although the micro-channel can be as deep as thethickness of the electrode layer, if the electrode is formed on anadjacent layer such as the electrolyte or separator. As an example, foran electrode 13 having a thickness “a” of 0.5 mm, an effectivemicro-channel could be on the order of 0.15 mm×0.15 mm height×depth.Comparatively, the prior art, depicted in FIG. 5, provides crossflowchannels 51 in metallic separators 52 having a thickness “b” on theorder of 3 mm, in which the height and depth “c” of the crossflowchannels are on the order of 1 mm×1 mm.

[0064] To begin operation of the electrochemical apparatus, the fuelcells 2 are heated by an outside heat source to near their operatingtemperature. Once the reaction is initiated, it sustains itself byproducing sufficient heat to support the ongoing cell operations. At thesame time, an electrical current flows through the stack by virtue ofthe oxygen ionization and neutralization within each cell. Thiselectrical current, driven by the oxygen potential difference, is theelectrical output energy. To produce useful quantities of electric powerhaving a useful voltage, fuel cells 2 of the type shown in FIG. 1 aretypically arranged in a series connected stack. Because each of the fuelcells 2 is so thin, up to hundreds of cells can be assembled in a singlestack of reasonable physical size.

[0065] Respectively, a gaseous fuel 22 is supplied to fuel manifold 19and an oxygen-bearing gas 24, such as air, is supplied to air manifold21. The oxygen-bearing gas flows through pores (and micro-channels, ifused) in the cathode 8, driven by the difference in the gas pressures inthe manifold and outside the cathode 8. The oxygen becomes negativelyionized in the cathode 8 at or near the electrolyte 10. The electrolyte10 is a good conductor of oxygen ions. Oxygen ions, thus, flow throughthe electrolyte 10 to reach the anode 12. At the anode 12, these ionsgive up their excess electrons to become oxygen atoms and molecules,fuel 22 flows through the porous anode (and micro-channels if used) andcombines with the oxygen to form water (and other products if fuelsother than hydrogen are used), releasing thermal energy.

[0066] At the stack level, the micro-channel cross-sectional area withineach cell 2 can also be varied from fuel cell to fuel cell to improvethe overall reactant balance within the stack. To illustrate, reactantsenter the stack at one end. The fuel manifold 19 has some finitepressure drop, so as reactant flows along the manifold, there is agradation in pressure from one end of the stack to the other. Foruniform electrodes, the gradient in pressure in the fuel manifold 19results in a differential flow across each anode 12. However, thecross-section of the micro-channels 26 can be tailored such that thepressure drop (or resistance to flow) across each anode 12 compensatesfor the pressure drop within the fuel manifold 19, thereby enablingconsistent reactant distribution from one end of the stack to the other.Reducing or increasing the number of micro-channels 26 can be used toproduce the same effect.

[0067] In a stack with reactants being fed from the top, the pressure ofreactants within the internal manifold will decrease progressivelytowards the bottom of the stack. To compensate for this decrease, thenet cross-sectional area of the micro-channels 26 in each cell withinthe stack can be progressively increased from top to bottom. Byincreasing the net cross-sectional area from top to bottom, a generallyeven distribution of reactants across the stack height will result. Toachieve a balanced distribution of reactants in other flow arrangements,for instance where fuel is fed from one end and oxygen bearing gas fromthe opposite end, the cross-sectional area of the micro-channels on theanode 12 and cathode 8 may be varied according to the direction of theflow. In a stack that receives fuel 22 from the bottom of the stack andoxygen bearing gas 24 from the top, the cross-sectional area of thecathode micro-channels in each cell would be increased from top tobottom, and the cross-sectional micro-channel area of the anode would beincreased from bottom to top to balance the distribution of reactantsacross the stack.

[0068] Balanced flow distribution of reactants reduces thermal gradientswithin the cell 2. Reactant depleted areas produce less heat thanreactant rich areas, thus, uniform reactant supplies across the cell 2and stack reduce the thermal gradients.

[0069] Cells 2 incorporating the varied micro-channel 26 are preferablysymmetrical about a central access. Oval, circular, or other symmetricalshapes offer good performance. Most preferably, the cell's major surfacewill have a circular shape with central feed holes. The central feeddesign facilitates uniform reactant flow distribution and allows highreactant utilization rates.

[0070] As can be appreciated, an almost infinite number of patternconfigurations are possible. It should further appreciated that whilethe above description is made with reference to a planar fuel cell, thepresent invention will include non-planar configurations including butnot limited to tubular fuel cells. Therefore, the above pattern ispresented as an example only and does not limit the scope of the claimedinvention.

[0071] Other embodiments of the solid oxide fuel cell and its componentsare disclosed in U.S. Pat. Nos. 5,445,903 and 5,589,285, assigned to thecommon assignee of the present invention, which patents are herebyincorporated by reference as if fully written out below.

[0072] It should now be apparent that various embodiments of the presentinvention accomplish the objects of this invention. It should beappreciated that the present invention is not limited to the specificembodiments described above, but includes variations, modifications, andequivalent embodiments defined by the following claims.

What is claimed is:
 1. An electrochemical apparatus comprising: at leastone cell; wherein the cell has a solid electrolyte disposed between anoxygen electrode and a fuel electrode, wherein at least one saidelectrode is a unitary electrode, at least one separator contacting thesurface of one of the electrodes opposite the electrolyte; wherein atleast one electrode defines a variable cross-section micro-channelpattern adapted to control flow distribution and pressure drop of atleast one reactant gas.
 2. The electrochemical apparatus of claim 1wherein at least one electrode defines a grid micro-channel pattern. 3.The electrochemical apparatus of claim 1 further comprising a pluralityof columns of electrode material extending from at least one surface ofat least one electrode or at least one surface of the electrolyte or atleast one surface of the separator; said columns defining themicro-channel pattern.
 4. The electrochemical apparatus of claim 1wherein the micro-channel cross-section narrows toward a rim of thecell.
 5. The electrochemical apparatus of claim 1 wherein themicro-channel pattern is formed within the electrode by at least one ofstenciling, screen printing, photolithography, ink spraying, pressingand calendering.
 6. An electrochemical apparatus comprising: at leastone cell, having a solid electrolyte disposed between oxygen electrodeand a fuel electrode; wherein at least one said electrode being aunitary electrode; and at least one separator contacting the surface ofone of the electrodes opposite the electrolyte; wherein the at least oneseparator defines a pattern of micro-channels; wherein the micro-channelcross-sectional area narrows toward the cell rim adapted to control flowdistribution and pressure drop of at least one reactant gas.
 7. Anelectrochemical apparatus comprising: a unitary electrode defining apattern of micro-channels, wherein the electrode has a rim; and saidmicro-channels have a cross-sectional area that decreases towards theelectrode rim for the controlled flow distribution and pressure drop ofat least one reactant gas.
 9. The electrochemical apparatus of claim 10wherein the micro-channel has a width of up to about 0.5 millimeter. 10.The electrochemical apparatus of claim 7 wherein the micro-channelpattern is formed within the electrode by at least one of stenciling,screen printing, photolithography, ink spraying, pressing andcalendering.